calixarenes are a well-known class of cyclic oligomers that are usually made by condensing formaldehyde with p-alkylphenols under alkaline conditions. V. Bohmer summarized the chemistry of calixarenes in an excellent review article (Angew. Chem., Int. Ed. Engl. 34: 713 (1995). Early transition metal complexes in which the four oxygen atoms of calix[4]arenes or O-methylated calix[4]arenes chelate to the metal are now known (see, e.g., J. Am. Chem. Soc. 119: 9198 (1997)).
Metal colloids constitute a group of compounds which have favorable properties as catalysts and catalyst precursors. In U.S. Pat. No. 4,144,191, a bimetallic carbonyl cluster compound catalyst for producing alcohols by hydroformylation is disclosed; either Rh2CO2(CO)12 or Rh3Co(CO)12 is used, bound to an organic polymer containing amine groups. The catalyst operates at low temperature and produces almost exclusively alcohols.
In the Finnish patent application No. 844634 the observation is made that a mixture of the monometal cluster compounds Rh4(CO)12 and Co4(CO)12 bound to an amine resin carrier serves as the extremely selective catalyst in producing alcohols. An advantage of the cluster mixture catalyst is that it is simpler to prepare and its activity can be optimized as a function of the mole proportion of the metals. When supported on inorganic oxide surfaces, iridium metal colloids in the form of clusters such as Ir4 and nanoparticles are active catalysts for olefin hydrogenation (Nature 415: 623 (2002)) and toluene hydrogenation (Journal of Catalysis 170: 161 (1997) and Journal of Catalysis 176: 310 (1998)). Besides olefin hydrogenation, iridium is in general used for a variety of catalytic processes that include propane hydrogenolysis, CO hydrogenation, toluene hydrogenation, decalin ring opening and related conversion of methylcyclohexane to dimethylpentanes (See Catalysis Letters 131: 7 (2009)), methanation, intramolecular hydroamination, asymmetric isomerization of primary allylic alcohols, allylic amination, hydroamination, hydrothiolation, C—H bond arylation of heteroarenes using iodoarenes, [2+2+2] cycloadditions, carbonylation of methanol, methane hydroxylation (See Chemical Communications 3270-3272 (2009)), and selective naphthenic ring opening without significant dealkylation of pendant substituents on the right (See U.S. Pat. No. 5,763,731).
It is known that the chemical properties of metal clusters such as catalytic activity or electronic properties such as electron binding energy vary depending on the size of cluster (aggregate of atoms) and the nature and number of ligands. It is further known that a critical limitation that prevents industrial application of metal clusters and, in general, metal colloid catalysts is lack of stability against aggregation (Gates et al., Nature 372: 346 (1994)). One method of dealing with lack of stability of metal clusters is to deposit them on a support such as a planar surface of an inorganic oxide or the interior microporosity of a zeolite. These surfaces can impart additional stability to metal clusters, and this has been demonstrated previously for Ir4 metal colloid species inside of zeolites even when decarbonylated (Gates et al., J. Phys. Chem. B 103: 5311 (1999), Gates et al., J. Am. Chem. Soc. 1999 121: 7674 (1999), Gates et al., J. Phys. Chem. B 108: 11259 (2004), and Gates et al., J. Phys. Chem. C 111: 262 (2007)). However, as ligands, zeolitic and inorganic oxide surfaces lack the ability to widely tune the catalytic and electronic properties of the cluster in large part because of the lack of available functional groups for interacting with the cluster (limited to be O, Si, and Al for zeolite), when compared with an organic ligand. In addition, it would be highly desirable to pattern discrete numbers of clusters in an organized spatial fashion relative to one another, because such organization can in principle also be used to affect catalysis. This is not possible to accomplish using the planar surface of an inorganic oxide or the interior microporosity of a zeolite as a template because more or less random deposition of cluster results throughout. The same is true when using the interior microporosity of a metal-organic framework material (See J. Materials Chem. 19: 1314 (2009)). Lithographic fabrication methods that have been used in the semiconductor industry have been used to prepare arrays of metal particles that are uniform in size, but these particles are typically larger than 100 nm in diameter (See Somorjai et al., Langmuir 14: 1458 (1998)). Recently, calixarenes have been successfully used as ligands to pattern up to eight cobalt colloids using the calixarene molecule as an organizational scaffold (See Vicens, et al., Dalton Transactions 2999-3008 (2009) and Wei et al., Chem Comm 4254-4256 (2009)). These colloids were synthesized via direct reaction of either Co2(CO)8 or Co4(CO)12 with alkyne-containing resorcinarene, under conditions that are identical to those used for non-calixarene ligands consisting of a single alkyne group. However, this type of direct reaction approach failed to synthesize a well-defined, characterizable set of products when reacting with the metal polyhedron, when using Co4(CO)12, and also fails at synthesizing calixarene-bound iridium colloids. An additional advantage when using a calixarene as ligand for a metal colloid is that the calixarene can be used to confine the nucleation and growth of the colloid during synthesis to be a small size via geometric restrictions and/or multivalency (See Wei et al., Chem Comm 4254-4256 (2009)). This type of confinement during metal colloid nucleation and growth has also been demonstrated previously using dendrimers as ligands for metal colloids (See Crooks et al., Accounts of Chemical Research 34: 181 (2001)); however, dendrimers do not allow control of patterning discrete numbers of less than eight colloids. The current invention offers the ability to pattern colloids in an organized assembly while also offering tenability of environment.
Some catalytic effects of transition metals complexed with calixarenes have been shown for olefin rearrangements [Giannini et al., J. Am. Chem. Soc. 121: 2797 (1999)], cycloadddition of terminal alkanes [Ozerov et al., J. Am. Chem. Soc. 122: 6423 (2000)] and hydroformylation [Csok et al., J. Organometallic Chem. 570: 23 (1998)]. The calixarenes in those investigations were coordinated with one or more metal cations that do not contain interactions between reduced metals as in a metal colloid. Calixarenes coordinated to metal cations that are grafted on oxide surfaces enforce isolation of the grafted metal cation by preventing aggregation into extended oxide structures [Katz et al., J. Am. Chem. Soc. 126: 16478 (2004)], [Katz et al., J. Am. Chem. Soc. 129: 15585 (2007)], and [Katz et al., Chem. Mater. 21: 1852 (2009)], and also afford the ability to tune catalysis of the grafted cation by virtue of the nature of coordinating groups as substituents on the calixarene skeleton [Katz et al, J. Am. Chem. Soc. 129: 1122 (2007)].
Coordinating a calixarene ligand to metal clusters offers numerous advantages including, but not limited to, more resiliency against aggregation due to the role of the calixarene as a sterically bulky barrier and, perhaps more importantly, opens the synthesis of new classes of highly tailorable functional materials, in which the calixarene serves as a nanoscale organizational scaffold for the assembly of complex active sites. The calixarene can also affect electron density on the metal colloid core by virtue of coordinating functional groups and substituents on the calixarene skeleton. In addition, metal colloids bound with calixarene contain void spaces either in between calixarenes on the surface or directly below the calixarene cavity, which can be used for binding and catalysis of molecules. All of the effects above have been previously demonstrated for calixarene-bound gold colloids [Ha et al., Langmuir 25: 10548 (2009)].
The continuing pursuit for smaller gold colloids that are stabilized with organic ligands is driven in large part by their use as building blocks for the assembly of functional materials in a variety of areas, such as drug and gene delivery ((a) Rivere, C., Roux, S., Tillement, O., Billotey, C., Perriat, P. Nanosystems for medical applications: Biological detection, drug delivery, diagnosis and therapy. Annales de Chimie-science des Materiaux, 31, 351-367 (2006) (b) Wang, G. L., Zhang, J., Murray, R. W. DNA binding of an ethidium intercalator attached to a monolayer-protected gold cluster. Anal. Chem. 17, 4320-4327 (2002) (c) Patra, C. R., Bhattacharya, R., Mukhopadhyay, D., Mukherjee, P. Application of gold colloids for targeted therapy in cancer. J.B.N. 4, 99-132 (2008)), biosensing ((a) Zhao, W., Chiuman, W., Lam, J. C. F., McManus, S. A., Chen, W., Yuguo, C., Pelton, R., Brook, M. A.; L1, Y. DNA Aptamer Folding on Gold Colloids: From Colloid Chemistry to Biosensors. J. Am. Chem. Soc. 130, 3610-3618 (2008) (b) Scodeller, P., Flexer, V., Szamocki, R., Calvo, E. J., Tognalli, N., Troiani, H., Fainstein, A. Wired-Enzyme Core-Shell Au Colloid Biosensor. J. Am. Chem. Soc. 130, 12690-12697 (2008). (c) Wang, L. H., Zhang, J., Wang, X., Huang, Q., Pan, D., Song, S. P., Fan, C. H. Gold colloid-based optical probes for target-responsive DNA structures. Gold. Bull., 41, 37-41 (2008)), nanofabrication ((a) Li, H. Y., Carter, J. D., LaBean, T. H. Nanofabrication by DNA self-assembly. Mater. Today, 12, 24-32 (2009) (b) Becerril, H. A., Woolley, A. T. DNA-templated nanofabrication. Chem. Soc. Rev. 38, 329-337 (2009) and references therein) and heterogeneous catalysis ((a) Choudhary, T. V., Goodman, D. W. Oxidation catalysis by supported gold nano-clusters. Top. Catal. 21, 25-34 (2002). (b) Turner, M., Golovko, V. B., Vaughan, 0. P. H., Abdulkin, P., Berenguer-Murcia, A., Tikhov, M. S., Johnson, B. F. G., Lambert, R. M. Selective oxidation with dioxygen by gold colloid catalysts derived from 55-atom clusters. Nature, 454, 981-U31 (2008). (c) Lee, S., Molina, L. M., Lopez, M. J., Alonso, J. A., Hammer, B., Lee, B., Seiferi, S., Winans, R. E., Elam, J. W., Pellin, M. J., Vajda, S. Selective Propene Epoxidation on Immobilized Au6-10 Clusters: The Effect of Hydrogen and Water on Activity and Selectivity. Angew. Chem., Int. Ed., 48, 1467-1471 (2009). (d) Hughes, M. D., Xu, Y.-J., Jenkins, P., McMorn, P., Landon, P., Enache, D. I., Carley, A. F., Attard, G. A., Hutchings, G. J., King, F., Stitt, E. H., Johnston, P., Griffin, K., Kiely, C. J. Tunable gold catalysts for selective hydrocarbon oxidation under mild conditions. Nature, 437, 1132-1135 (2005). (e) Haruta, A. When gold is not noble: Catalysis by colloids. Chemical Record, 3, 75-87 (2003).). Previous research has been done on the catalytic activity of metal clusters on solid supports. Xu Z et al., Nature, 1994, 372: 346-348; and Argo et al., Nature, 2002, 415: 623-626. To the extent that these references disclose metal clusters on a metal oxide support that in some sense might be considered a ligand, the references disclose metal clusters complexed to only one ligand.
In these applications small gold colloids are advantageous from the perspectives of penetrating into confined spaces such as intracellular compartments that are inaccessible with larger colloids offering greater surface-to-volume and electronic tenability via choice of surface ligands enabling assembly of materials at a higher resolution and information density and exhibiting preferred catalytic properties that are different from those of bulk. The passivation of small metal colloids with organic ligands decreases their ubiquitous tendency to aggregate into larger and more stable colloids. The ideal for such a passivating layer is to facilitate two at first site mutually incompatible functions: (i) stabilize a small metal colloid, while also (ii) offering access to the metal surface for ease of binding and conjugation to other molecules. Large (4 nm) gold colloids that are bound with bulky calixarene ligands have been previously shown to have greater stability against aggregation and sintering, tunable electron density via interactions with coordinating calixarene substituents, and accessible metal surfaces that serve as small-molecule binding sites located between adsorbed ligands (Ha J M, Solvyov A, Katz A, Synthesis and characterization of accessible metal surfaces in calixarene-bound gold colloids. Langmuir, 25, 10548-10553 (2009) and references therein). All of the effects above have been previously demonstrated for calixarene-bound gold colloids [Ha et al., Langmuir 25: 10548 (2009)].
Quite surprisingly, we have discovered that complexation of metal atoms with a ligand, e.g., a calixarene, allows the formation of metal colloids smaller than those previously produced. Moreover, counterintuitively, it has been discovered that the smaller colloids have a greater fraction of surface atoms accessible than larger colloids. As exposed metal surface atoms are an important element of the chemical, e.g., catalytic and adsorptive (binding) properties of the metal colloids, this discovery increases the utility and versatility of metal colloids.
Previously known gold clusters include those that have been encapsulated in cucurbituril. Corma A et al., Chem. Eur. J., 2007, 13: 6359-6364. The encapsulated ligands, however, were inaccessible to cyanide anion as a gold leaching agent. The stoichiometry of these enapsulated clusters would be one ligand per gold core. Using cucurbiturals with smaller cavity sizes led to larger gold clusters (4 nm), but with metal cores considerably larger than the size of the ligand.
Nowicki A et al., Chem. Commun., 2006, 296-298; Denicourt-Nowicki A et al., Dalton Trans., 2007, 5714-5719 (Denicourt-Nowicki I); and Denicourt-Nowicki A et al., Chem. Eur. J., 2008, 14: 8090-8093 (Denicourt-Nowicki II) investigated cyclodextrin-complexed ruthenium nanoparticles. The histogram in FIG. 1 of Denicourt-Nowicki I shows all metal cores to be larger than the size of the beta-cyclodextrin used as ligand. This is consistent with the schematic in Nowicki labelled as Scheme 2, which suggests a larger Ru(0) core surrounded by smaller cyclodextrin ligands.
Sylvestre J-P et al., J. Am. Chem. Soc., 2004, 126: 7176-7177 describe the preparation of gold particles using cyclodextrin ligands akin to Denicourt-Nowicki I. Again, as in Denicourt-Nowicki I, the size of the metal core is larger than the cyclodextrin and ranges from 2-2.5 nm.
Goldipas K R et al., J. Am. Chem. Soc., 2003, 125: 6491-6502 disclose nanoparticle-cored dendrimers that, according to the authors, consist of Au cores larger than the dendritic wedge that comprises the ligand to which they are complexed, thus making encapsulation not physically possible.